Quantifying the effect of SBS molecular structure on the upper service temperature rheological properties of modified binders

Abstract

At upper service temperatures, rutting is one of the common ways through which asphalt pavements fail. The use of polymer modified binders (PMBs) significantly reduces the rutting tendency in asphalt pavements. Commercially, styrene–butadienestyrene (SBS) polymers with different molecular structures are used extensively for asphalt binder modification. Modified asphalt binders were prepared using four commercial-grade SBS polymers having different molecular structures (Linear, radial, high vinyl, and diblock) and at SBS content of 3, 4.5, and 7 wt%. Frequency sweep measurements (0.01–100 rad/s) were performed on the modified asphalt binders at 60 °C. Frequency sweep analysis reveals that the influence of molecular structure on the upper service temperature rheological properties is predominantly observed at lower frequencies (≤0.1 rad/s). At 0.01 rad/s frequency, the divergence in complex modulus and phase angle among the SBS modified binders with the four SBS polymers with different structures were ≈ 500 % and ≈ 25°, respectively. While at 10 rad/s, the variation in complex modulus and phase angle was only 60 % and 5°, respectively. The higher molecular weight (Mw), triblock structure, rigid polystyrene segments at the chain ends, and long radial branches in radial SBS polymer results in a stronger polymer network in the binder. The lower Mw and diblock structure in diblock SB polymer resulted in a weaker polymer network in the binder. The result implies that the rheological signature of the SBS polymer and its structure was predominantly observed at lower frequencies (<0.1 rad/s). Irrespective of the four SBS polymers, the correlation (R2) of the rheological properties with rutting was enhanced significantly at frequency ≤ 0.1 rad/s. This study illustrates the importance of lower frequencies in quantifying the effects of the SBS structure on the upper service temperature rheological properties of SBS-MABs. For a good correlation, effective grading, and quality control, the rheological variable at frequency ω ≤ 0.1 rad/s is important in modified asphalt binders.